Laser Diodes (LDs) based on III-nitride semiconductors are commercially available with emitting wavelengths in the UV, blue and green portions of the electromagnetic spectrum. Such devices are used, for example, in illumination and display applications. It is particularly important to provide such devices with high electrical and optical performances.
A III-nitride (or also nitride) semiconductor structure based on GaN and its alloys including In and Al (hereafter referred to as (Al,In,Ga)N alloys) can be made to form high efficiency LD devices. The semiconductor layer structure for such devices can be manufactured by forming semiconductor layers sequentially on a substrate. These layers are physically connected and generally obtained with high crystal quality using epitaxial deposition or growth method such as Metal Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE). FIG. 1A shows such a conventional structure of a laser diode device as described by Nakamura S and Fasol G, The Blue Laser Diode, p. 293, 1997 (Berlin: Springer). The structure includes an n-side semiconductor region 113, active region 106 and p-side semiconductor region 114 formed on a substrate 112. The deposition direction or the growth direction is defined by the direction perpendicular to the semiconductor layers surface of this structure. The substrate is made of sapphire.
Under application of a low electrical excitation to this structure, charge carriers such as electrons and holes will move across the structure and recombine radiatively in the active region 106 resulting in emission of photons. The wavelength of the emitted photons and thus of the light emission from the LD device is determined by the bandgap of the active region.
Semiconductor laser diode devices as described above require confinement of the emitted photon and carrier recombination to an active region and a higher electrical excitation. Additionally, an optical cavity established by reflecting mirror surfaces at both ends of the device induces an optical amplification. Once these requirements are satisfied lasing operation can be achieved.
Confinement of the emitted photons can be achieved by the utilization of a separate confinement heterostructure. With continued reference to FIG. 1A, this may be achieved by arranging a n-guide layer 105 below and a p-guide layer 108 above the active region 106, each of the n-guide layer and the p-guide layer having a refractive index higher than the effective index of the guided light. Further confinement is achieved by arranging a n-cladding layer 104 below and a p-cladding layer 109 above the active region 106, each of the n-cladding layer and the p-cladding layer having a refractive index lower than the effective index of the guided light. The upper p-guide layer 108 and upper p-cladding layer 109 form part of the p-side semiconductor region 114. The lower n-guide layer 105 and lower n-cladding layer 104 form part of the n-side semiconductor region 113. The n-side semiconductor region 113 additionally includes buffer layer 101, n-contact layer 102, and a buffer layer 103.
This structure may lead to strong confinement of the emitted photons in a transverse direction, therefore propagating parallel to the active region. Between the active region 106 and upper p-guide layer 108, a carrier blocking layer 107 is provided, which has a wider band gap compared to adjacent layers.
The p-cladding layer 109 has a shaped upper surface that may be formed by post-growth processing. In a typical layout, the shaped upper surface of the p-cladding layer 109 is a ridge formed in the plane of growth, which enhances light confinement to the direction of the longest dimension of the ridge.
A p-contact layer 110 is formed on top of the upper p-cladding layer 109. Electrical contact is made to the p-contact layer 110 by a metal electrode layer 111a formed thereon, which allows the device to be electrically activated. Injection of charge carriers to the p-side semiconductor 114 only occurs in the area defined by the metal electrode layer 111a and the p-contact layer 110 which coincide with the shaped ridge. An electrode metal layer 111b is also formed on the surface of the n-contact layer 102, which can be achieved by etching through the semiconductor layer structure described herein by post growth processing and depositing a metal electrode layer on the n-contact layer 102.
At either end of the ridge, a surface perpendicular to the growth direction and the direction of the longest dimension of the ridge is defined by a process such as cleaving or etching. Light escaping by transmission through these surfaces is used for the intended applications. In this described arrangement, the device is said to be edge emitting.
FIG. 1B shows the active region 106 of the semiconductor laser emitting device of FIG. 1A. The active region 106 is designed to confine recombination of charge carriers (electrons, holes), formed of a triple quantum well structure where semiconductor layers of low bandgap (quantum well layers 115) are arranged between layers of higher band gap (quantum barrier layers 116. The layers of the active region are formed of InGaN alloy semiconductor material. The band gap of InGaN material decreases as the amount of In increases. FIG. 1C shows the amount of In x in the composition for each quantum well layer 115 and the amount of In y in the composition for each quantum barrier layer 116. FIG. 1D shows the corresponding energy levels for electrons and holes. The energy E0 indicates the band gap of the quantum well layers 115.
Such quantum well structures are highly desirable for localizing recombination of carriers. However, in III-nitride material systems there is an issue of non-uniformity in carrier concentration across the quantum wells and this has an impact on the laser diode performance. For example, quantum wells 115 close to the n-side semiconductor region 113 have generally a much higher concentration of electrons than quantum wells 115 close to the p-side semiconductor region 114 under a given operating condition. Analogously, quantum wells closer to the p-side semiconductor region have a higher hole concentration than quantum wells close to the n-side semiconductor region. Non-uniformity in hole concentration is particularly pronounced due to their lower mobility as compared with electrons in this material system.
In addition, for achieving a laser diode device with emission wavelength longer than UV (>405 nm), it is typically necessary to decrease the bandgap of the quantum wells 115 in the active region 106. This has a consequence of increasing the confinement of electrons in the quantum wells 115 closer to the n-side semiconductor region 113 and holes in the quantum wells 115 closer to the p-side semiconductor region 114. This further contributes to increased non-uniformity of carriers across the quantum wells.
Lasing light emission is achieved when radiative recombination rate from carrier recombination in the active region reaches a level which is able to compensate for optical losses. This corresponds to a carrier concentration under a high electric excitation also known as threshold current. At this threshold, lasing will be achieved and the quantum wells contributing to this lasing process will be the one(s) with the highest radiative recombination rate. If non-uniformity exists among the quantum wells 115, the lasing process will be achieved at higher electric excitation and therefore at a higher threshold current.
In Nakamura S and Fasol G, The Blue Laser Diode, p. 201-221, 1997 (Berlin: Springer), non-uniformity in carrier distribution may be reduced by reducing the number of quantum well layers to, for example, one. However, in such an arrangement, a single quantum well layer is insufficient to localize all electrons injected to the active region 106 and significant flow of electrons is not confined in the active region and electrons are injected into the p-side semiconductor region 114. This leads to recombination of electrons and holes in the p-side semiconductor 114, in particular in the upper guide layer 108, thereby reducing injection of holes to the active region 106 and causing increase in threshold current. To reduce this overflow effect, a single quantum well structure with increased quantum well layer thickness may be used to increase the confinement of electrons in the quantum well layer. However, it is difficult to achieve InGaN based quantum well layers with high crystal quality when the thickness of these layers is increased. This has a consequence to increase defects in the quantum well layers which may be responsible for increasing non-radiative recombination rate and further increasing the threshold current of the laser emitting device.
In U.S. Pat. No. 9,123,851B2 (Goda et al, Sep. 1, 2015), the non-uniformity in carrier concentration of a multi quantum well structure may be addressed by reducing the thickness of quantum barrier layers 116 which are arranged between quantum well layers 115. However, in such structure, because the mobility of electrons is higher than that of the holes, the non-uniformity of electron concentration is enhanced and results in increased overflow of electrons to the p-side semiconductor region 114, in particular in the upper p-guide layer 108. This has a consequence to increase the recombination of electrons and holes in the p-guide layer and reduce the injection of holes to the active region. The laser light emitting device achieved with such structure may exhibit an increased threshold current.
In JP4622466B2 (Koji, Mar. 3, 2005), non-uniformity of carrier concentration across the quantum wells may be reduced by lowering the band gap energy of the barrier layers 116 compared to quantum well layers 115 to enhance transport of carriers through the barrier layers 116. However, in this case, transport of electrons past the barrier layers is improved to a greater extent than the transport of holes past the barrier layers, thus overall increasing overflow of electrons to the p-side semiconductor region 114, in particular in the upper p-guide layer 108.
Reducing the bandgap energy of the barrier layers 116 can be achieved by increasing the amount of In in the barrier layers 116 but this degrades the crystal quality of the active region, leading to higher levels of non-radiative recombination and increasing the threshold current of the laser diode device.
In Zhang et al. (Journal of Applied Physics 2009 105:2), improved performance may be achieved by reducing the absorption loss of photons in upper p-guide layer 108, lower n-guide layer 105, upper p-cladding layer 109, and lower n-cladding layer 104. This can be achieved by reducing the concentration of dopant species in these layers, which however negatively impacts hole injection from A-side semiconductor region and n-side semiconductor region into the active region.
There remains a problem of reducing threshold current of a III-nitride based semiconductor laser diode by improving carrier injection and carrier uniformity to the active region while maintaining strong light confinement and high crystal quality, and without degrading the characteristics of emitted laser light.